1. Field of the Invention
The present invention relates to an optical scanning lens, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In an optical scanning device widely known with reference to a laser printer, a digital copier, and so forth, a beam emitted from a light source is deflected by a light deflector, the deflected beam is condensed toward a surface to be scanned by a scanning and imaging optical system, a beam spot is formed thereby on the surface to be scanned, and thus, optical scanning of the surface to be scanned is performed.
Recently, a plastic-made (made of plastic) optical scanning lens is employed as the scanning and imaging optical system or as a part thereof.
The plastic-made optical scanning lens is formed through plastic mold. As this type of lens can be easily mass-produced, it can be manufactured at low costs. Accordingly, by employing the plastic-made optical scanning lens, it is possible to effectively reduce the costs of the scanning and imaging optical system, and, thereby, the costs of the optical scanning device. Further, with regard to the plastic-made optical scanning lens, it is possible to easily form a special lens surface shape such as an aspherical surface through plastic mold. Accordingly, it is possible to simplify the scanning and imaging optical system (reducing the number of lenses required) and/or to improve optical performance thereof.
However, this type of plastic-made optical scanning lens has a problem in that a non-uniform distribution of the refractive indexes occurs inside of the plastic-made optical scanning lens.
Through plastic mold, molding is performed in which thermally molten plastic material is injected into a die, and is cooled in the die. At this time, cooling begins from a part of the material in contact with the die. Accordingly, the center of the plastic material is cooled slowly relative to the periphery thereof. Thereby, a non-uniform distribution of density (the density at a part cooled rapidly becomes higher than that at a part cooled slowly) inside of the plastic. As a result, the distribution of the refractive indexes of the thus-formed lens is not uniform inside thereof. As the density of the periphery of the formed lens is higher than that of the center thereof, the refractive index is lower at the lens center while, the nearer to the surface the position thereof becomes, the higher the refractive index of the formed lens becomes, in general.
When the distribution of the refractive indexes of the plastic lens is measured by a method described later, the variation in refractive index is like an approximately quadratic curve along each of a lens optical axis direction, a main scanning direction and a sub-scanning direction.
The plastic-made optical scanning lens is designed assuming that the distribution of refractive indexes inside thereof is uniform. Accordingly, when the plastic lens has a non-uniform distribution of refractive indexes inside thereof, it cannot exhibit the performance according to the design. Specifically, defocus occurs such that a position of imaging of the deflected beam differs from the surface to be scanned, thereby, a position of beam waist of the deflected beam is changed from the surface to be scanned, and, as a result, the diameter of the beam spot increases.
In order to reduce such a non-uniform distribution of refractive indexes inside of the lens, it can be considered to cool the molten plastic material in the die, very slowly for a long time (for example, for ten and some hours) in a thermostatic chamber. However, by such a method, the productivity of the optical scanning lens becomes worse, and the manufacturing costs thereof increase. Accordingly, the advantage of the plastic-made lens such as requiring low costs may be cancelled.